Publication date: 8th July 2026
For wearable and implantable health monitoring, near-infrared (NIR) light is attractive
because its low absorption in biological tissue allows deep penetration. Such
applications require flexible, efficient, and stable light sources. Colloidal
semiconductor nanocrystals (NCs) are well suited for this purpose because they
combine solution processability with efficient NIR emission. Among them,
CuInS2/ZnS core/shell NCs are a promising RoHS-compliant alternative to
conventional heavy-metal-based quantum dots. CuInS2/ZnS NCs emit via defectmediated
recombination involving confined hole states [1], but the role of hole
injection and transport in populating these states remains unclear.
In this work, we study the effect of the hole transport layers (HTL) on the efficiency
and stability of NIR CuInS2/ZnS LEDs. We compare several HTL materials (TFB,
Poly-TPD, TAPC and CBP) as well as double HTL stacks with different energy levels
and hole mobilities. We find that a double HTL consisting of 10 nm CBP and 30 nm
TAPC gives the best performance in both maximum efficiency and operational
stability, achieving an EQE of 2.5%, close to the theoretical limit expected from the
10% PLQY in solution, while retaining 60% of its initial luminance after 10 h of
operation at 50 mA cm-2. To understand the improved performance of the double
HTL, we studied how it affects charge injection, transport, and accumulation in the
device. We used hole-only devices to isolate hole transport and injection behavior.
Capacitance–voltage and capacitance–frequency measurements were performed to
probe charge accumulation and interface effects. In addition, electroluminescence
and transient electroluminescence measurements were used to analyze
recombination dynamics and device operation under bias. Correlations observed
within the datasets provide valuable insights into charge injection, transport, and
recombination dynamics, offering a deeper understanding of the mechanisms driving
device performance and stability.
This work highlights the importance of HTL engineering for CuInS2/ZnS LEDs,
increasing the EQE from 1.3% for the best single HTL to 2.5% using an optimized
double-HTL architecture. We expect that our results can be applied also to other
RoHS-compliant quantum dots. These advances support the development of reliable
NIR light sources for future biomedical sensing applications.
This work was financially supported by the Deutsche Forschungsgemeinschaft (DFG, German research foundation) through GRK 2767 (451785257). S.L. thanks the Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP) and the Institute of Applied Physics for providing access to their laboratory facilities for device fabrication.
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